Filtering a substrate through a skein of fibers has long been known in the art. Irrespective of the configuration of the skein, whether arcuate or vertical; and if vertical, whether in a rectangular or cyclindrical configuration, to date, the emphasis has been on maintaining the outer surfaces of the fibers clean by backwashing them (through their lumens) frequently, either with permeate, or clean water when the substrate is aqueous. Because backwashing with substrate would foul the lumens of the fibers, use of substrate as flushing liquid was never considered. Ser. No. 09/024,417 taught that the most effective flushing fluid was a mixture of gas (typically air) and a flushing liquid such as permeate, clean water or substrate, preferably fresh, clean substrate. Except for a brief period before flushing was commenced, flushing liquid was introduced into the aerator and soon thereafter the two-phase mixture was introduced so that it provided scrubbing gas to the skein essentially continuously.
Since it was believed that the continuous flow of scrubbing bubbles was required to be maintained, interrupting the flow of scrubbing bubbles for a short period as little as 5 min, was not considered a viable option. This belief proved untenable as it was subsequently found that interrupting aerating gas for as much as 10 min, and flushing the aerator with flushing liquid alone during the interruption, preferably for less than 5 min, effectively cleaned the aerator provided it was designed for the purpose.
An orifice through which air exits the lateral walls of an aerator, becomes clogged because solids penetrate the interior of the aerator and are deposited on its inner walls forming a sludge or slime which gradually accumulates on the periphery of the orifice. As air continuously passes through the orifice, the sludge is dried, providing a base for the accumulation of yet more sludge which, in turn, is dried, until the orifice is substantially plugged. As the diameter of the orifice is reduced, the size of the bubbles decreases in size, but they also rise more slowly than coarse bubbles, resulting in diminished effectiveness in scouring the essentially vertical, smooth surfaces of the fibers.
Using the method taught herein, a skein of fibers, most preferably in the vertical orientation, is kept continuously operational at equilibrium flux for long enough to be economical in commercial installations, despite having flow of scrubbing bubbles temporarily interrupted. By frequently interrupting flow of gas to a fouled scrubbing aerator only briefly, and flushing the aerator with a specified mass flow of flushing liquid, it has been found that deposits loosened from within the aerator are directly returned to the substrate but may be later removed. Gas flow to the scrubbing aerator is promptly re-established. The orifices are kept clean, which in turn keeps the fibers clean, and filtration economical.
Aerators which require cleaning during use are mainly used in the treatment of municipal sewage as described in U.S. Pat. No. 3,153,682 to Walker, inter alia. However, the purpose of such aeration is to provide mixing of the contents of a tank while aerating its contents, not to produce coarse bubbles for the specific purpose of scrubbing fibers. Further, such aeration produces a circulating roll of the contents in the tank (see col 1, lines 11-13). Air was supplied through large orifices in combination with a stream of water against an impingement surface so the air is broken up into fine bubbles and "clogging of the air orifices is virtually never encountered" (see col 1, lines 36-43). To maintain the desired clean orifices for satisfactory operation an orifice velocity of 126 ft/sec (38.4 meters/sec) with a 7" water column head loss, is specified (see col 3, lines 23-27), stating that it is desirable not to let the velocity fall below 70 ft/sec (21.34 m/sec). Though perhaps justifiable in an aerator for municipal sludge, the pressure drop at such high velocity would render a vertical skein inoperable over an extended period. Moreover, when the air supply was cut off, as when the power supply is interrupted, sludge would enter orifices in the aerator.
More preferable is the use of a box-shaped aerator such as shown in FIG. 2 herein, having an opening in the bottom as a water-seal leg; alternatively a horizontal tubular aerator is provided with openings in the bottom so that liquid entering the tube runs back into the substrate. The drawback to this method is that an air/liquid interface is maintained within the aerator, and in the case of liquids with a tendency to foam, the foam builds up in the aerator. It is this mechanism which leads to fouling of coarse bubble aerators because the foam collecting above the line of air orifices, eventually thickens and hardens to block flow of air.
In his later U.S. Pat. No. 3,242,072 Walker emphasized the importance of maintaining a velocity of 70 ft/sec to prevent "creeping in" of the liquid sewage, and resultant growths' (see col 3, lines 62-64). To ensure adequate margin for maintaining the required air pressure Walker provided a pressure leg, down-flush leg, or blow-off (identified by numeral 31 in FIGS. 1, 2 and 4). To clean the aerator, Walker stops operation to fill risers 16 with liquid to the level of the liquid in the tank so that the entire air pressure is initially available to move the water and force it through a passage in blow-off 31, so that the water flows fast with a scouring effect (see col 4, lines 23-28).
Later, in U.S. Pat. No. 3,501,133 to Drier et al, they again shut off the air supply to flush sludge out of their spargers 24 (see col 4, lines 20-43). To avoid discharging sludge through the orifices 31 they provided "down-flush tubes" extending from the low point of each sparger. As before, the spargers were positioned to contribute to the same over-all rolling movement of the tank's contents being careful not to lose the characteristic of isolated air-lift columns which provided aeration efficiency.
In light of the specific problem of keeping the orifices of a scrubbing aerator of particular design clean, it is not surprising that the prior art did not consider how using only a flushing liquid to keep such a scrubbing aerator in a sufficiently clean condition to provide the necessary uniform flow of scrubbing bubbles. Specifically, the many references teaching microfiltration with a skein of fibers, failed to suggest how to clean clogged orifices in an internally fouled aerator scrubbing aerator, and to do so in situ, when the stream of bubbles was briefly interrupted.
Flushing an aerator with a single phase flushing liquid, whether filtered substrate, or fresh, clean substrate, or water, particularly where the aerator is designed to generate bubbles in an amount and in a size range specifically directed to scrub a skein of fibers while the skein is operating to withdraw permeate, would not be suggested because no prior art reference used an aerator of the specified design to aerate substrate while that substrate was being filtered.